1. Field of the Invention
This invention relates to pressure sensors for detecting the pressure of measurement fluids, which can be utilized as pressure sensors of electrostatic capacitance type utilizing changes in the electrostatic capacitance between opposed electrodes for pressure detection.
2. Description of the Related Art
FIG. 18 shows a general prior art pressure sensing element 900 which is used for an electrostatic capacitance type pressure sensor (first prior art example, see Japanese Patent Publication No. S60-34687).
This pressure sensing element 900 comprises a thick base 901 and a thin diaphragm 902 which can be deformed by the pressure of a measurement fluid. The base 901 and diaphragm 902 are disposed parallel and spaced apart by a predetermined distance by a ring-like spacer 903. Between the opposed surfaces 904 and 905 of the base 901 and diaphragm 902, respectively, a space 906 is defined such that it is surrounded by the spacer 903.
The opposed surface 904 of the base 901 is provided with a substantially circular electrode 901. The opposed surface 905 of the diaphragm 902 is also provided with a substantially circular electrodes 911 of the same size as the electrode 910. The electrode 910 and 911 which are opposed by each other form a capacitor 912 with electrostatic capacitance C.
The surface of the diaphragm 902 opposite the opposed surface 905 is a pressured surface 907, to which the pressure of the measurement fluid is applied.
From a portion of the edge of the electrode 910 extends a continuous electrode lead conductor path 913 toward an outer end portion. Also, from a portion of the edge of the electrode 911 extends a continuous electrode lead conductor path 914 toward the outer end portion.
Dashed loops in the drawing represent electric lines of force produced between the electrodes 910 and 911. The electric lines of force are straight in the neighborhood of the center of the electrodes 910 and 911, but near the edge thereof they are disturbed by the conductor paths 913 and 914 and spacer 903.
In this first prior art example, atmospheric air or the like is introduced into the space 906 to set up a reference pressure, while the pressure of the measurement fluid is acted from the pressured surface 907. The diaphragm 902 is thus flexed with the differential pressure. With the flexing of the diaphragm 902, the distance between the electrodes 910 and 911 is changed to change the electrostatic capacitance C of the capacitor 912. This is utilized for detecting the pressure of the measurement fluid. The space 906 may be made vacuum for absolute pressure measurement.
In such first prior art example, for detecting the pressure of the measurement fluid as gauge pressure, it is necessary to introduce atmospheric air into the space 906. However, the dielectric constant of the atmospheric air in the space 906 is subject to changes with changes in the temperature and relative humidity of atmosphere, thus causing changes in the electrostatic capacitance C of the capacitor 912. This makes it impossible to obtain accurate pressure detection. In addition, the insulation resistance between the electrodes 910 and 911 are changed with changes in the temperature and relative humidity of atmosphere, thus again making it impossible to obtain accurate pressure detection.
Further, the electric lines of force produced near the edge of the electrodes 910 and 911 are convex outward as shown in FIG. 18, and the electrostatic capacitance C of the capacitor 912 is influenced and changed by temperature and relative humidity changes, and hence resultant dielectric constant changes, of the materials of the base 901, diaphragm 902 and spacer 903 which are found in the outwardly convex portion of electric lines of force. Further, when an object 920 with an extremely different dielectric constant approaches the diaphragm 902, the electric lines of force become convex toward the object 920, and this affects the electrostatic capacitance C.
Furthermore, since the electrode lead conductor paths 913 and 914 extend outward from the edge of the electrodes 910 and 911, the electric lines of force are caused to become convex outward so as to affect the electrostatic capacitance C and make it impossible to obtain accurate pressure measurement.
FIGS. 19 to 21 show a pressure sensing element 800, which is an improvement over the general pressure sensing element 900 as the first prior art example (second prior art example, see Japanese Patent Laid-Open Publication No. 60-56233).
FIG. 19 is a side sectional view of the element 800, and FIGS. 20 and 21 are plan views showing the element 800 in disassembled states. The shaded portions in FIGS. 20 and 21 are not sections but are made so for facilitating the explanation.
The pressure sensing element 800 is similar in construction to the above first prior art example. That is, a thick base 801 and a thin diaphragm 802 are parallel and spaced apart by a predetermined distance by a ring-like spacer 803. A space 806 surrounded by the spacer 803 is formed between the opposed surfaces 804 and 805 of the base 801 and diaphragm 802, respectively.
On the opposed surface 804 of the base 801 are provided a substantially circular electrode 810 and a partly missing ring-like (i.e., C-shaped) electrode 811 surrounding the electrode 810. On the opposed surface of the diaphragm 802 is provided a substantially circular electrode 812 which has the same outer diameter as that of the outer electrode 811.
The electrodes 810 and 812 which oppose each other form a capacitor 814 with electrostatic capacitance C1, while the electrodes 811 and 812 form a capacitor 815 with electrostatic capacitance C2.
The surface of the diaphragm 802 opposite the opposed surface 805 serves as a pressured surface 807, to which the pressure of the measurement fluid is applied.
From a portion of the edge of the electrode 810 extends a continuous electrode lead conductor path 816 through the missing zone of the electrode 811 toward an outer end portion. From a portion of the outer edge of the electrode 811 extends a continuous electrode lead conductor path 817 toward an outer end portion.
Meanwhile, from a portion of the edge of the electrode 812 extends a continuous electrode lead conductor path 818 toward an outer end portion.
The base 801 has a communication hole 819 communicating the space 806 and the outside. Atmospheric air providing the reference pressure is introduced into the space 806 through the communication hole 819.
In such second prior art example, like the above first prior art example, atmospheric air is introduced into the space 806 to provide the reference pressure, while the pressure of the measurement fluid is acted from the pressured surface 807, thus causing the diaphragm 802 to be flexed with the differential pressure. With the flexing of the diaphragm 802 the distance between the electrodes 810 and 812 and the distance between the electrodes 811 and 812 are changed to change the electrostatic capacitances C1 and C2 of the capacitors 814 and 815. This is utilized for detecting the pressure of the measurement fluid.
In this second prior art example, the electrostatic capacitances C1 and C2 of the capacitors 814 and 815 are affected by changes in the dielectric constant .epsilon. in the space 806 caused with changes in the temperature, relative humidity, etc. of the space. However, the changes are compensated for through detection of C1/C2 with a measure circuit (not shown), thus permitting accurate pressure measurement. Denoting the inter-electrode distances of the capacitors 814 and 815 by D1 and D2 and the electrode areas of the capacitors by S1 and S2, EQU C1/C2=(.epsilon..times.S1/D1)/(.epsilon..times.S2/D2)=(S1.times.D2)/(S2.tim es.D1)
The dielectric constant .epsilon. is thus canceled, thus permitting the pressure measurement by compensation for the influence of the changes in the dielectric constant .epsilon..
In this second prior art example, however, the electric lines of force that are produced between the outer electrode 811 on the side of the base 801 and the electrode 812 on the side of the diaphragm 802, are influenced and disturbed by the electrode lead conductor paths 817 and 818 and spacer 803. This leads to inconvenience similar to that in the above first prior art example. That is, the electrostatic capacitance C2 of the capacitor 815 is changed with the temperature and relative humidity changes, i.e., accompanied dielectric constant changes, of the materials of the base 801, diaphragm 802 and spacer 803 that are found in the outwardly disturbed portion of the electric lines of force. It is thus impossible to obtain accurate pressure detection.
Further, the outer electrode 811 on the side of the base 801 is partly missing ring-like (i.e., C-shaped) so that the conductor path 816 of the central electrode 810 can extend through the missing portion. This arrangement has end effects on the electrostatic capacitance C2 of the capacitor 815, thus making it impossible to obtain accurate pressure detection.
Further, the electrode 811 is partly missing ring-like and has the conductor path 817 extending outward, and also the substantially circular electrodes 810 and 812 have respective conductor paths 816 and 818 which also extend outward. That is, either electrode is not so simple in shape. This dictates a complicated process of manufacture.
FIG. 22 shows a prior art pressure sensing element 700 used for an electrostatic capacitance type pressure sensor (third prior art example, see Japanese Patent Laid-Open Publication No. S59-148842).
The pressure sensing element 700 comprises a thick base 701 and a thin diaphragm 702 which can be deformed by the pressure of the measurement fluid. The base 701 and diaphragm 702 extend parallel and spaced apart by a predetermined distance by ring-like high-melting glass 703 and low-melting glass spacers 704. A space 707 surrounded by the high- and low-melting glass spacers 703 and 704 is defined between the opposed surfaces of the base 701 and diaphragm 702.
On the opposed surfaces 705 and 706 of the base 701 and diaphragm 702, respectively, respective electrodes 710 and 711 are provided such as to form a capacitor 712 with electrostatic capacitance C.
From the electrodes 710 and 711 leads 712 and 713 are led through the base 701 such as to extent outward from the back 709 of the base 701 (i.e., the surface opposite the opposed surface 705). The back 709 is also provided with a vacuum seal member 714 for vacuum sealing the space 707.
The surface of the diaphragm 702 opposite the opposed surface 706 serves as a pressured surface 708, to which the pressure of the measurement fluid is applied.
In this electrostatic capacitance type pressure sensor using the pressure sensing element 700 as the third prior art example, the space 707 is vacuum sealed, while the pressure of the measurement fluid is acted from the pressured surface 708, thus causing the diaphragm 702 to be flexed. With the flexing of the diaphragm 702 the distance between the electrodes 710 and 711 is changed to change the electrostatic capacitance C of the capacitor 712. This is utilized for detecting the pressure of the measurement fluid.
With this third example of the pressure sensing element 700, in which the base 701 and diaphragm 702 are joined to each other by the high- and low-melting glass spacers 703 and 704, the accuracy of the gap between the base 701 and diaphragm 702 can be secured with the accuracy of printing of the high-melting glass 703. In addition, reliable bond seal can be obtained by printing the low-melting glass 704 after the printing formation of the high-melting glass spacer 703.
With this third example of the pressure sensing element 700, however, a relative humidity increase causes reduction of the creepage surface resistance of ceramics or like material of the base 701 and diaphragm 702 and the high- and low-melting glass as the joining spacer members, thus causing a current leak between the electrodes 710 and 711. In this case, accurate pressure measurement can not be obtained.
Further, since it is necessary to suppress the generation of such a leak current, the distance from the edge of the electrodes 710 and 711 to the high- and low-melting glass spacers 703 and 704 has to be increased to increase the creepage distance of insulation and creepage surface resistance. This leads to a size increase of the pressure sensor or makes it impossible to use large area electrodes. Therefore, the capacitance changes are reduced with the same inter-electrode distance change, making it impossible to obtain accurate measurement.
Referring to FIGS. 19 to 21 again, in the second prior art example of the pressure sensor 800, the electrodes 810 to 812 are led to the outside by connecting leads 826 to 828 extending from them toward outer end portions.
In such electrostatic capacitance type pressure sensing element 800 as the second prior art example, in addition to the case of leading the electrodes to the outer end portions through the leads 826 to 828, there is a case of leading the electrodes to the back of the base (i.e., the surface opposite the surface with the electrodes thereon) by forming through holes in the base and forming conductive portions in the through holes (see Japanese Patent Publication No. S63-9174 and Japanese Utility Model Laid-Open Publication No. S57-105943).
However, in the above second prior art example shown in FIGS. 19 to 21, the electrodes 810 to 812 have to be led out to the outside by providing a portion of the outer edge of each of the electrodes 810 to 812 with each of the conductor paths 816 to 818 extending toward outer end portions and connect the leads 826 to 828 to these conductor paths 816 to 818. Further, the path for leading atmospheric air into the space 806 to provide the reference pressure of the pressure measurement is constituted by a communication hole 819, which is provided quite separately from the structure of leading the electrodes 810 to 812 to the outside. Therefore, the process of manufacture is complicated, and it is impossible to reduce the cost of manufacture.
Further, where the base is provided with through holes for leading electrodes therethrough as noted above, these through holes are used exclusively for the lead-out of the electrodes, and a separate hole has to be provided to secure the path for introducing atmospheric air into the space. Therefore, like the second prior art example shown in FIGS. 19 to 21, the process of manufacture is complicated, and it is impossible to reduce the cost of manufacture.
An object of the invention is to provide a pressure sensor, which permits accurate measurement of the pressure of the measurement fluid and also can be manufactured readily and reliably.